Though successful in Europe, it was dismissed in the United States. Argento admitted in the book Broken Mirrors, Broken Minds: The Dark Dreams of Dario Argento that he was less than pleased with the film, and has repeatedly cited it as his least favorite of all of his films.[7]

Franco "Cookie" Arnò (Karl Malden), a middle-aged blind man, is out at night walking with his niece Lori (Cinzia De Carolis) when he overhears a man in a parked car mention blackmail. After Franco and Lori return home, the man in the car gets out and breaks into a large medical complex, the Terzi Institute. The following day, the police and reporter Carlo Giordani (James Franciscus) investigate the break-in, Carlo introducing himself to Franco during a run-in.

Meanwhile, Dr. Calabresi (Carlo Alighiero) looks at his files in his office and phones someone who agrees to meet with him. Calabresi tells his fiancee, Bianca Merusi (Rada Rassimov), that he knows who broke into the institute and what was taken, but doesn't wish to tell anyone yet, saying it could mean a "big step forward". At a train station, while a group of reporters are waiting for a celebrity to arrive by train, someone approaches Calabresi and pushes him onto the tracks. Lori reads the newspaper for Franco about Calabresi's "accidental death", describing the picture and telling him that Giordani wrote the article. The two of them go to see the reporter at his office and ask if the picture has been cropped. Carlo calls up Righetto (Vittorio Congia), the paparazzi photographer who snapped the picture. Righetto goes back to the original and sees a moving hand in the far left of the frame. As Righetto prepares to print the photograph, he is strangled to death with a cord. The killer takes the photo, along with all the negatives, and leaves. Carlo, Franco, and Lori arrive and find the body, calling the police led by Chief Investigator Spimi (Pier Paolo Capponi).

Carlo and Franco survey the Institute from a distance, the former looking through a pair of binoculars while describing the people leaving the building to Franco: Mombelli, Esson, Casoni, and Braun, as well as Professor Fulvio Terzi (Tino Carraro) and his daughter Anna (Catherine Spaak). Carlo goes to the Terzi home and expresses his desire to talk about Calabresi's "accident". Afterwards, Carlo speaks with Anna, and he evades her questions of what he and her father spoke about. The two drive away together, but soon realize they are being followed by police and drive at full speed to evade them.

Meanwhile, Franco and Lori go to talk with Bianca, and she says that she couldn't find anything in the house relating to her fiance's death. At a local restaurant, Anna tells Carlo about the institute's research of "chromosome alteration" and "XYY", the extra Y producing a "criminal tendency" in a person. Carlo goes to see Dr. Braun (Horst Frank) at the St. Peter's Club and talks to him about someone being after the institute's secret drug, news that doesn't seem to vex the doctor.

Bianca takes a taxi to Calabresi's parked car in a lot. Inside, she finds a tiny note with the details of his fatal appointment at the station. She tapes the note to the inside of her locket. Bianca calls Franco and says she knows who killed Calabresi, but will only tell him in person. As Bianca returns to her apartment, the killer attacks and strangles her with a cord. The killer rummages through her purse, but doesn't find anything. Franco shows Carlo a note he received in which the killer threatens them. Carlo tells Franco he found out that Casoni was fired from his last job, and Braun received a lot of money. Carlo goes to see Casoni and the doctor talks about the institute's "wonder drug" and the "XYY pattern". Carlo then asks Dr. Mombelli about XYY, and the doctor says that everyone in the institute was tested, but their results are confidential.

The killer approaches Carlo's front door and injects two milk cartons, dropped off by the local milkman, with a syringe. Carlo arrives home and brings the milk cartons inside. Anna arrives shortly thereafter and they talk about more about the research, as well as her results of the XYY test. They end up having sex. Afterwards, Carlo pours a glass of milk from one of the cartons when Franco phones saying that someone tampered with the gas line on his stove, flooding his apartment with methane gas and also may try to kill Carlo. Carlo notices the milk that had bled through the hypodermic needle holes and knocks the glass away from Anna before she can drink it.

The following day, Carlo meets with one of his old friends and informants, Gigi (Ugo Fangareggi), for help in investigating the Terzi break-in which may have been an inside job. Carlo and Gigi break into Terzi's house and discover that Anna is adopted and (via a diary) that Terzi "adored" the woman. Carlo goes to the police station and learns from Spimi that Bianca often met with Braun and that the cops cannot find the doctor. Carlo runs a story in the newspaper about Braun being a suspect in the break-in, and a former gay lover of Manuel (Werner Pochath) (Braun's new lover) approaches Carlo with information on where Braun is hiding. Carlo goes over to the apartment where he's attacked by Manuel. Carlo wins the fight, and sees Braun lying dead on the couch.

A few days later, Franco contacts Carlo about Bianca's locket and suggests that the note that she found might still be there. Franco and Carlo head to the cemetery and open Bianca's family crypt. Carlo gets her coffin open while Franco waits by the door. Carlo finds the locket and discovers the note behind a metal plate and hands it to Franco. As Carlo closes the coffin, the killer shuts the crypt door, locking him inside, and attacks Franco. The killer takes the note, but Franco stabs him with his walking cane (which has a knife hidden inside it). While Franco reopens the door to let Carlo out, Lori is hit on the head by the killer and put in the back of a car. Franco and Carlo find the taxi which the killer rode in and discover blood on the back seat. The killer calls Franco and tells him to stop investigating the break-in/murders or otherwise he will kill Lori.

Carlo goes to the police to report the kidnapping and they go to the Terzi house. During their visit, Anna comes downstairs with a cloth wrapped around her hand. Carlo tells Anna he knows about her incestuous relationship with her adoptive father and expresses suspicion about the milk incident (Anna had the glass of poisoned milk for some time without drinking it). But Anna claims that she only cut her hand on a broken vase and was nowhere near the cemetery. Then, Terzi arrives and confirms her story.

Carlo and the police arrive at the Terzi Institute and search the place for Lori, but they find nothing. Carlo sees blood dripping from the ceiling in one room. He climbs up to the roof and finds Casoni, who hits him in the face and kicks him to the ground. Casoni, with a stab wound to his stomach, goes to a back room where a bound and gagged Lori is and prepares to stab her. Carlo runs in and tackles Casoni, but is stabbed in the chest. The police arrive on the roof and chase Casoni. Franco stops him with his cane blade and Casoni confesses that he murdered Calabresi and the others to cover up that he tested positive for the XYY chromosome and that Calabresi was attempting to blackmail him. When Franco asks about Lori, Casoni tells Franco that he killed her. Enraged, Franco swings his cane at Casoni, knocking him through a sky window and down an elevator shaft to his death, just as a now-free Lori calls out for Franco.

Dario Argento and Dardano Sacchetti together mapped out the plot for The Cat o' Nine Tails, and split the writing of the screenplay between them.[8] However, because the production was set up on the basis of the first 40 pages of the script, and those pages were all written by Argento, Argento demanded that he receive sole screenplay credit. Being credited for story alone meant a substantial pay cut for Sacchetti, so this set off a bitter and publicized dispute between Sacchetti and Dario and Salvatore Argento (the film's producer, and Dario's father).[8]

The Cat o' Nine Tails was shot between September and October 1970.[9] The film was shot on location in Berlin, Turin, and at Cinecitta Studios in Rome.[9]